CAIRO, April 23 (Reuters) - When French Egyptologist Olivier Perdu saw a fragment of a pharaonic statue on display in a Brussels gallery last year, he assumed it was a twin of an ancient masterpiece he had examined in Egypt a quarter of a century earlier.

The reality was an even more remarkable coincidence: the fragment was part of the very same artefact - a unique 6th century B.C. statue hewn from pale green stone - that Perdu had received special permission to study in Cairo in 1989.

The statue, a 29 cm-high (11 inches) representation of a man wearing a pharaonic headdress and holding a shrine to Osiris, the god of the afterlife, was smashed by looters who broke into the Cairo Museum during the 2011 uprising against Hosni Mubarak.

Its top portion had been missing since then.

"I was just astonished," Perdu told Reuters. "Through examining all the stains and irregularities I could conclude that it was indeed the same piece.

"What I had between my hands in Brussels was the object that I had studied in the Cairo museum in 1989."

Thanks to his chance encounter, the piece excavated in 1858 has found its way back to Egypt. Horrified to learn he had purchased a stolen artefact, the buyer offered to surrender it right away, Perdu said. It is now back in Cairo, where conservation experts have reunited it with the rest of the statue.

Antiquities theft has flourished in Egypt in the three years of chaos since the 2011 uprising, robbing this ancient civilisation of an indeterminate amount of heritage stolen from museums, mosques, storage facilities, and illegal excavations.

A small group of government employees tasked with scouring the internet in search of stolen treasures put up for sale has seen its work increase dramatically following the antiquities crime wave that accompanied the political upheaval of 2011.

In a few cases, thanks to serendipity, experts have spotted Egyptian artefacts in auction houses and private collections in the West and worked for their repatriation.

But while Egypt has recovered about 1,400 artefacts to date, it faces a struggle to get back all that has been lost.

There is no record of just how many antiquities have gone missing. Many were taken from illegal digs, and there is no way to know that they even exist.

"Most of them are not registered, because they were excavated by criminal gangs, not by specialists," said Ahmed Sharaf, head of museums at Egypt's antiquities ministry.

Swathes of the desert are now pockmarked with unauthorised digs, where thieves have used shovels and backhoes in search of buried treasure. Some have even dug tunnels to break into untapped antiquities sites without attracting attention.

Though officials claim improved security is curbing the theft, pieces continue to go missing, even from well-guarded sites. Just this month, two ancient statues were stolen from a storage facility at Luxor temple in southern Egypt.

"In the last three years, the sale of stolen antiquities has flourished - inside Egypt and abroad," said Sharaf.

ANTIQUITIES LAUNDERING

It's not just pharaonic artefacts that are being stolen.

Muslim rulers dating back to the seventh century left their mark on Egyptian architecture, and since 2011 thieves have torn decorative pieces out of mosques and other Islamic monuments in central Cairo.

Thieves have also taken big items such as doors and pulpits, perhaps with the complicity of guards, say officials. One of the problems, says Sharaf, is that the religious endowments ministry does not adequately guard mosques still used for public worship.

Thanks again to chance, some Islamic artefacts have been spotted in Western art markets. Last year, wooden panels ripped out of a 13th century mausoleum in Cairo in 2012 turned up at London auction house Bonhams.

Islamic art scholar Doris Behrens-Abouseif identified the origin of the eight carved wooden panels after Bonhams asked her colleague to inspect their inscriptions. The falsified ownership documents provided by the would-be seller did not add up.

"These pieces were very clear and could be easily identified," Behrens-Abouseif told Reuters, citing their unique historical inscription. Saved from the auction block, the panels remain with Bonhams pending the outcome of a police inquiry.

Bonhams said it had taken custody of the panels for research purposes but never entered them into a sale.

Officials can only speculate about how such items make their way out of Egypt and into Western markets. One smuggling route is thought to run through Egypt's Sinai Peninsula - where traffickers in drugs and people also operate - to Israel and across the Mediterranean to Europe.

Officials tell stories of antiquities being wrapped in carpets, stowed inside inexpensive replicas, or hidden in shipments of vegetables.

The international police agency Interpol says it has little information about how stolen antiquities reach foreign shores, but says they are often "laundered" through other countries.

"Sometimes it's very difficult to know exactly when these items arrived to the legal market and in which way, because it's possible they pass through different countries, different private selling, before reappearing in a legal auction," Interpol officer Fabrizio Panone told Reuters.

THE REPO UNIT

In a cramped office in the upscale Cairo neighbourhood of Zamalek, five government employees spend their days browsing online trading sites and Western auction house catalogues looking for stolen antiquities.

Ali Ahmed, who heads the unit, says his work has increased massively in the last three years. He trained as an archaeologist, but today likens his role to that of a detective.

With Egypt facing an unprecedented problem, he wants tighter restrictions on the trade in Egyptian antiquities in the West.

Crucially, anyone seeking to sell an artefact of Egyptian origin should be required to produce a document showing it was lawfully exported from Egypt, whose laws permitted the trade in antiquities until 1983, when all such trade was banned.

Stolen artefacts often pick up export certificates in transit countries, easing their transport to market countries.

"I know that every Egyptian artefact is made in Egypt. Prove to me that you obtained it legally," said Ahmed.

He achieved some success this year when online auction site eBay agreed to remove from its site antiquities which he believed had been looted in Egypt.

"We monitor what is sold in public. The problem is what is sold in secret," Ahmed said.

"I hope the civilised world cooperates with us. When we talk about Egyptian heritage, it isn't just for Egyptians but for all of humanity." (Additional reporting by Shadia Nasralla; Editing by Michael Georgy and Giles Elgood)